It is known to one skilled in the art that water-in-oil emulsions are difficult to stabilize against coalescence of water droplets and thus against water separation due to the tight packing of the water droplets, which is a consequence of the usually high proportion of disperse phase (>65%). A further consequence of the high packing density is a per se high viscosity of the emulsion.
In order to achieve a stability required by the cosmetic industry, stabilizing waxes and sometimes also high-viscosity oils are generally used, both of which increase the viscosity of the (external) oil phase, thus reducing the mobility of the water droplets and protecting against coalescence. High-viscosity oils and stabilizing waxes, however, have an adverse effect on the feel on the skin; in particular, these types of oils and waxes induce a heavy and sticky feeling on the skin. Although, in principle, it is possible to switch to oil-in-water emulsions, which, as is known, have a lighter feel on the skin, as alternatives, these types of emulsions have a significantly lower care effect than oil-in-water emulsions.
It is thus a generally desirable aim to make water-in-oil emulsions as light as possible, i.e., low viscosity, like oil-in-water emulsions, without having to sacrifice their particular care effect. In particular, one object is to permit water-in-oil emulsions to have the lowest possible viscosity, which are nonsticky and nevertheless stable. A particular challenge in this area is the formulation of oil-in-water emulsions which are of such low viscosity that they are sprayable. In this connection, the emulsifier to be used plays a particular key role.
W/O (water-in-oil) emulsifiers which are based on natural raw materials are of considerable interest for ecological reasons both to manufacturers and also to the consumers of emulsion preparations. These are, in particular, emulsifiers which contain no polyethylene glycol-containing radicals (“PEG-free” emulsifiers). For this reason, and despite their often average performance, partial esters of polyalcohols, such as glycerol, polyglycerol, sorbitol or methyl glycoside, and fatty acids, such as oleic acid or isostearic acid, continue to be used widely.
This type of emulsifier is not suitable, for example, for flowable emulsions (lotions) and for creams with a high content of natural triglycerides. The creams corresponding to the stability requirements of the market (thermal stability from −15° to +45° C., sometimes from −25° to +50° C.) consist of lipid-like components, predominantly paraffin oils and fatty acid esters of monoalkanols (MW<500); these have more favorable technological properties than the higher molecular weight triglycerides. Nevertheless, for stabilization, relatively high concentrations of viscosity-increasing waxes (≧3%) are required; however these have an adverse effect on the application properties since they produce an undesired sticky-greasy feel on the skin.
The polyglycerol esters of di- and polymerized unsaturated C18-fatty acids have considerably better emulsification properties than the polyalcohol fatty acid partial esters. These polyglycerol esters are obtained from the mono- and diglycerides of vegetable oils, preferably soya oil, by thermal treatment for a number of hours at about 300° C. or by the transesterification of a thermally polymerized vegetable oil with polyglycerol.
The polyglycerol polyricinoleates formed by an analogous method from castor oil are likewise high-performance W/O emulsifiers (see, for example, DE-B-44 09 569).
On account of their sensitivity to oxidation and the sometimes marked greasy-rancid odor, this class of substance has hitherto only become established in a few cosmetic or pharmaceutical emulsion preparations. The massive thermal stress during production and the unsaturated character (e.g., iodine number about 100) are primarily responsible for this.
Polyglycerol polyhydroxystearate, which is chemically related to polyglycerol polyricinoleate and can likewise be prepared from vegetable raw materials, is likewise used as a W/O emulsifier. Polyglycerol polyhydroxystearates have a higher oxidation stability and thus an increased sensory quality (see, for example, DE-C-44 20 516).
However, both classes of substances are only able to form relatively low-viscosity W/O lotions with an oil phase content of >22% by weight. On account of this minimum amount of oil phase and on account of the rather rich sensory character of these emulsions, the formulation of stable W/O emulsions with a very light feel on the skin is not possible or is only possible to a very limited degree. Added to this is the fact that emulsifiers of this type, particularly when formulating triglyceride-containing formulations, often have problems with low-temperature stability, which in many cases can only be solved by adding coemulsifiers.
The improvement in the freeze-thaw stability is of considerable practical interest for the transportability and storability of emulsion preparations. As a result of prolonged storage at very low temperatures, or as a result of extreme temperature fluctuations during extended transportation, the inadequate emulsion stabilization can become apparent from significant water separations in emulsion preparation or even lead to the complete breaking of the emulsion.
EP-B-0 835 862 describes polyglycerol partial esters obtainable by esterification of a polyglycerol mixture with a degree of esterification of the polyglycerol between 30 and 75% and saturated or unsaturated, linear or branched fatty acids having 12 to 22 carbon atoms and dimer fatty acids with an average functionality of from 2 to 2.4. These esters have an additional advantage over polyglycerol polyhydroxystearate in that the freeze stability of emulsions containing the polyglycerol partial esters is significantly improved. However, the emulsions are still relatively thick, for which reason these polyglycerol partial esters are primarily suitable for producing rich lotions and creams.
An object of the present invention is therefore to develop a PEG-free emulsifier which permits the formulation of low-viscosity emulsions that have a pleasantly light feel on the skin and at the same time the required freeze-thaw stability.